1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming an image, which has a function for forming an image on a special sheet such as a tab sheet by an electrostatic system or an electrophotographic recording system.
2. Related Background Art
A conventional color image forming apparatus employs a transfer mechanism in which an image developed on a photosensitive drum is transferred to an intermediate transfer belt, and the image is further transferred from the intermediate transfer belt to a recording sheet. In this transfer mechanism, the image on the intermediate transfer belt is pulled (or attracted) by a transfer roller provided on a back surface of the recording sheet, thereby transferring the image on the recording sheet. According to this transfer mechanism, since images can stably be transferred irrespective states of sheets such as warpage or end portions of sheets, the transfer mechanisms are widely employed in color image forming apparatuses.
According to such a transfer mechanism, however, when an image is transferred from the intermediate transfer belt by a transfer roller, if the recording sheet does not exist on a transfer portion, the image is adversely transferred onto the transfer roller and as a result, there is a problem that a back surface of a recording sheet which is fed to the transfer portion next is contaminated. To solve such a problem, in the case of recording sheets having regular sizes, it is possible to prevent the transfer roller from pulling the image when the recording sheet does not exist on the transfer portion by controlling voltage of the transfer roller. When an image is to be formed on a tab portion of a tab sheet (tab is located at a rear end of the tab sheet), it is necessary to maintain the voltage of the transfer roller so that the image is attracted toward the transfer roller at least until the rear end of the tab portion passes through the transfer portion. When the tab portion of the tab sheet exist on the transfer portion, however, the tab portion is sandwiched between the intermediate transfer belt and the transfer roller, but in the portion of the sheet other than the tab, there is nothing between the intermediate transfer belt and the transfer roller. Therefore, when an image whose tab position is not correct is placed on the intermediate transfer belt, there is a problem that the transfer roller is contaminated. When an original image is copied on a tab sheet, if the original is not a tab sheet but a sheet having a regular size, such a problem is prone to be generated.